An optical waveguide device using electro-optic crystal, such as a substrate made of lithium niobate (LiNbO3) or LiTaO3, is fabricated by forming an optical waveguide through depositing a metal layer on the crystalline substrate and thermal diffusing the metal layer or through proton exchange in benzoic acid after patterning, and then forming an electrode in the vicinity of the optical waveguide.
Forming an electrode in electro-optic crystal in which such an optical waveguide is formed to provide light propagating through the optical waveguide with a variation in refractive index, the electro-optic crystal is configured to serve as an optical device that performs optical modulation. Further, in the optical device equipped with such an electrode, an insulating layer, such as SiO2 buffer layer is interposed between the electrode and the substrate in order to avoid light absorption by the electrode.
The electro-optic modulator made from a ferroelectric material such as lithium niobate (LiNbO3) has already been put into practice in the field of an optical communication system and others. In addition, a high-speed optical modulator is being put into market which is capable of modulating with a high-frequency electric signal as high as 40 GHz.
FIG. 23 is a top view of a normal optical device used as the above optical modulator, and FIG. 24 is an AA′ sectional view of the optical device 100 of FIG. 23. In the optical device 100 of FIGS. 23 and 24, a Mach-Zehnder optical waveguide 102 serving as a Mach-Zehnder interferometer (MZI) is formed on a Z-cut substrate (hereinafter simply called LN substrate) 101 formed from lithium niobate, and a signal electrode 104 and a grounding electrode 105 are also formed via a buffer layer 103. The Mach-Zehnder optical waveguide 102 includes two branch waveguides 102b-1 and 102b-2 and Y-shaped waveguides 102a and 102c that are coupled to the edges to the two branch waveguides 102b-1 and 102b-2 and that respectively splits and couples propagating light.
The signal electrode 104 is formed so as to include the superior region of one of the two branch waveguides 102b-1 and 102b-2, and the grounding electrode 105 is formed so as to enclose the signal electrode 104 on the substrate 101, keeping a predetermined distance from the signal electrode 104. In addition, application of a voltage from the signal electrode 104, for example, shifts the phase difference between light components propagating through the branch waveguides 102b-1 and 102b-2, so that an optical signal, the intensity of which is on-off modulated is output from the Y-shaped waveguide 102c serving as a coupling waveguide.
FIG. 27 is a diagram depicting a variation (an optical modulation intensity curve) in optical output level with a voltage applied to the signal electrode 104. As depicted in FIG. 27, the level of the optical output varies with the applied voltage to draw a sine wave. For example, under the previous setting of the operation point voltage to be V0, optical modulation can be accomplished by varying an applied voltage in the range of R in the drawing through the use of the operation point as a reference.
The optical device 100 serving as such an optical modulator is known for influence of a phenomenon called temperature drift that the operation point voltage varies in accordance with a variation in temperature on modulation characteristics. Here, detailed description will be made in relation to temperature drift. The output light intensity S is represented by the following formula (1) under the assumption that the optical device illustrated in FIG. 23 has a configuration in which the Y-shaped waveguides 102a and 102c are 3 dB splitting couplers and light is not lost in the propagation process of the Mach-Zehnder optical waveguide 102.
                    S        =                              S            0                    ⁢                                    COS              2                        ⁡                          (                                                                    ϕ                    2                                    -                                      ϕ                    1                                                  2                            )                                                          (        1        )            
where, the symbols φ2 and φ1 represent the phase shifts that light undergoes while propagating through the branch waveguides 102b-1 and 102b-2, respectively, and are each represented by formula (2).
                    ϕ        =                                            (                                                n                  0                                +                                  Δ                  ⁢                                                                          ⁢                  n                                            )                        ⁢            L                    =                                    (                                                n                  0                                -                                                                            n                      0                      3                                        2                                    ⁢                  rE                                            )                        ⁢            L                                              (        2        )            
Where, n0 represents the refractive index of the substrate, r represents an electro-optic coefficient, E represents the intensity of an electric field applied to the waveguide and is proportional to a voltage applied to the electrode, and L represents the length of interaction between the electrode and the waveguide. In other words, the light intensity varies with the difference in refractive index between the two waveguides 102b-1 and 102b-2 in the optical device 100 serving as an MZI optical modulator, as illustrated in FIG. 23. As described above, an optical modulator sets a predetermined operation point on a certain light intensity curve in advance and carries out optical modulation using the operation point as a reference. Here, assuming that temperature generates a difference in refractive index between the two waveguides 102b-1 and 102b-2, the light intensity can be expressed in formula (3) in which the voltage to shift to a predetermined operation point and the modulating voltage are respectively represented by Vb and Vs.S=S0 COS2(A(Vb+Vs)+BΔn(T))  (3)
Where, the symbols A and B are constants of proportion; Δn (T) represents a difference in refractive index between the two branch waveguides 102b-1 and 102b-2 caused by temperature. Vb usually represents a DC voltage, and Vs represents a high-speed RF signal used for driving the modulator. Generation of Δn(T) due to temperature means deviation of the operation point set by Vb. Therefore, temperature drift means deviation of the operation point due to a variation in temperature and one of the reasons of temperature drift is appearance of a difference in refractive index between branch waveguides 102b-1 and 102b-2 which difference depends on temperature.
The major causes of occurrence of temperature drift are first major cause due to an asymmetric voltage which is caused by charge generated in accordance with temperature when the substrate is formed of a material having a pyroelectric effect exemplified by LiNbO3 and which affects two waveguides, and the second major cause due to a stress caused by a difference in the thermal expansion coefficient between the electrode and the substrate.
The following Patent References 1 and 2 disclose techniques to resolve temperature drift caused by the above first major cause. Specifically, forming a layer contacting to the electrodes makes the charge symmetric, so that electric fields penetrating the optical waveguides are equalized, which electric fields are generated by charge occurring the substrate material in response to a variation in temperature.
In addition, Patent References 3-5 below disclose techniques to resolve temperature drift caused by the above second major cause. In order to ensure a high-speed operation, the electrode of an optical modulator needs to be as high as several dozen μm, which generates a large stress due to a difference in thermal expansion coefficient between the electrode and the substrate to cause temperature drift. The techniques disclosed in Patent References 3-5 form two optical waveguides constituting the MZI relative to the electrode configuration to be symmetric as much as possible in attempt of reducing temperature drift caused by the stress.    [Patent Reference 1] Japanese Examined Patent Application Publication No. HEI 5-78016    [Patent Reference 2] Japanese Patent No. 2873203    [Patent Reference 3] Japanese Patent Application Publication No. 2001-255501    [Patent Reference 4] Japanese Patent Application Publication No. 2002-122834    [Patent Reference 5] Japanese Patent Application Publication No. 2006-84537
However, in a circumstance where an optical waveguide needs to be formed so as to be deviated toward one side of the substrate 101 like the optical waveguide 102′ constituting the MZI of FIG. 25, a stress S caused by chip distortion due to temperature (distortion caused by warp of the substrate 101) or a difference in thermal expansion coefficient between the buffer layer 103 and the substrate 101 applies different stresses to the two optical waveguides 102b-1 and 102b-2 and thereby generates temperature drift.
For example, when two optical modulators are concurrently formed on a signal substrate (i.e., a single chip) as disclosed in a reference of “Masaharu DOI et al., the institute of communication, spring of the year 2004, C-4-43”, the optical modulators are deviated toward respective sides of the substrate. The techniques of the Patent References 1-5 have difficulties in compensating temperature drift occurring in the above case.
FIG. 26 is an AA′ sectional view of FIG. 25. A stress generated by a variation in temperature of the substrate 101 illustrated in FIG. 25 is schematically along the direction S in FIG. 26, for example. Since a waveguide closer to either chip edges (either edge in the width direction of the substrate 101) is affected by a larger stress, the branch waveguide 102b-1 closer to the chip edge is affected by a relatively larger stress than that affects the branch waveguide 102b-2. The difference of stress affecting the waveguides results in asymmetric variation in refractive index, which causes temperature drift.